In https://github.com/haskell/cabal/pull/2439 the invocation of Haddock is
changed to use Haddock's new `--package-name` and `--package-version` flags,
necessary for GHC >= 7.10 (Cabal issue
https://github.com/haskell/cabal/issues/2297 / Haddock issue
https://github.com/haskell/haddock/issues/353). However, this commit also
removes the `-package-name` argument to GHC. This is incorrect, as it means
that GHC will end up calling the package `main` and we end up with Haddock link
environments such as

    (trans_H9c1w14lEUN3zdWCTsn8jG:Control.Monad.Trans.Error.strMsg, main:Control.Monad.Error.Class)

Note that before this commit we ended up with

    (trans_H9c1w14lEUN3zdWCTsn8jG:Control.Monad.Trans.Error.strMsg, mtl-2.1.1:Control.Monad.Error.Class)

which is equally wrong as it uses a package source ID rather than a package key
(Haddock issue https://github.com/haskell/haddock/issues/362). Instead, we need
to pass _both_ `--package-name` and `--package-version` to Haddock, and
`-package-name` or `-this-package-key` to GHC, depending on the version.
Thankfully the infrastructure for chosing between `-package-name` and
`-this-package-key` is already in place, so we just have to make sure to
populate the `ghcPackageKey` field. After this commit the link environment
looks like

    (trans_H9c1w14lEUN3zdWCTsn8jG:Control.Monad.Trans.Error.strMsg, mtl_Koly6qxRZLf86guywd4tkE:Control.Monad.Error.Class)

which is correct.